A large nuclear spin polarization p,, is desirable for most NMR investigations since NMR is basically an insensitive method and since the signal-to-noise ratio is proportional to p.. The classical method to polarize nuclear spins is the "solid effect" in which the much higher Boltzmann polarization of unpaired electrons in doublet ground states is transferred to the nuclear spins by inducing forbidden transitions with the selection rules Ams = & 1, Am, = & 1. However, a drawback of this method is the presence of unpaired electrons within the samplc which produce unwanted effects in most experiments to be performed with polarized nuclear spins. Using the unpaired electronic spins of optically excited triplet states instead of doublet ground states has the important advantage that after switching off the exciting light it renders a diamagnetic crystal with polarized nuclear spins due to the short lifetime of the triplet states. -The population differences of the electronic sublevels of the excited triplet states to be transferred to the nuclear spins originate in this case from the very selective population rates s, and depopulation rates k, due to the selection rules of spin orbit coupling. Since the transfer occurs by inducing forbidden transitions by microwaves, the method was termed Microwave Induced Optical Nuclear Polarization (MI -ONP). The different CW-and pulse-techniques are discussed. Furthermore, a more reccntly developed technique will be described where the polarization is obtained with special cross-relaxation transitions which satisfy the "Hartmann-Hahn condition" for electronic spins spinlocked in the rotating frame to the radio frequency &field and nuclear spins in the static magnetic field Bo in the laboratory frame. -The highest nuclear polarization obtained so far is p . = 33% which corresponds to an amplification factor of more than 2000 as compared to the Boltzmann polarization in the same magnetic field, but we expect to reach even higher polarizations of more than 50% with improved techniques, in particular with higher B,-fields.
